Coccidiosis is a disease caused by infection with one or more of the many species of coccidia which is a subdivision of the phylum Protozoa, intracellular protozoal parasites of the subphylum Apicomplexa and the genus Eimeria. The genus Eimeria contains the species of major economic importance in domestic birds, such as chickens, ducks, geese, guinea fowl, peafowl, pheasants, pigeons and turkeys. While coccidiosis occurs in practically all kinds of birds, the parasites are host specific and each species occurs in a single or in a limited group of related hosts. On the other hand, avian hosts are known to harbor more than one species of coccidia. Species of Eimeria that cause coccidiosis in chickens include E. acervulina, E. brunetti, E. hagani, E. maxima, E. mitis, E. mivati, E. necatrix, E. praecox and E. tenella. E. acervulina is one of the most common species found in the litter of broiler houses. It has a great reproductive potential and is regarded as pathogenic because it produces a marked depression in gain of body weight, higher feed conversion and it produces gross lesions in the upper small intestine.
Among domesticated birds, chickens are the most susceptible to significant economic losses from coccidiosis, although losses can also occur within turkeys, geese, ducks, and guinea fowl. Coccidiosis has also produced serious losses in pheasants and quail raised in captivity. The effects of a coccidiosis infection can take the highly visible form of devastating flock mortality, but another undesirable effect is morbidity and/or weight loss which results from infection.
During the life cycle, the Eimeria parasite passes through a number of stages (see, e.g., U.S. Pat. No. 6,100,241 for an overview). The life cycle begins when the chicken ingests the infectious stage, known as the sporulated oocyst, during ground feeding or by inhalation of dust. The wall of the sporulated oocyst is ruptured by a combination of mechanical grinding action and chemical action in the gizzard and intestinal tract, resulting in the release of four sporocysts. The sporocysts pass into the duodenum where they are exposed to bile and digestive enzymes resulting in the release of two sporozites per sporocyst.
The sporozoites are mobile and search for suitable host epithelium cells in order to penetrate and reproduce in them. Following infection of an epithelium cell, the parasite enters the schizont phase of its life cycle, producing from 8 to 16 to >200 merozoites per schizont. Once released from the schizont, the merozoites are free to infect further epithelium cells. After two to five of these asexual reproduction cycles, the intracellular merozoites grow into sexual forms known as the female or macrogametocyte and the male or microgametocyte. Following fertilization of the macrogametocyte by the microgametes released from the microgametocyte, a zygote is formed which creates a cyst wall about itself. The newly formed oocyst is passed out of the infected chicken with the fecal droppings.
With the correct environmental conditions of temperature and humidity and sufficient oxygen in the air, the oocyst will sporulate into the infectious stage, ready to infect a new host and thereby spreading the disease. Thus, no intermediate host is required for transfer of the parasite from bird to bird.
The result of the Eimeria parasite infecting the digestive tract of a chicken may be a reduction in weight gain, increased feed conversion, cessation of egg production and, in some cases, death. The increase in intensive production of poultry has been accompanied by severe losses due to this parasite; indeed, coccidiosis has become an economically important parasitic disease.
In the past, several methods have been used in attempts to control coccidiosis. Prior to the advent of chemotherapeutic agents, improved sanitation using disinfectants, together with the mechanical removal of litter, was the main method employed; sufficient oocysts, however, usually remained to transmit the disease. The introduction of coccidiostatic agents in the feed or drinking water, in addition to good management, resulted in some success at disease control. Such agents have been found to suffer from a drop in effectiveness over the years, due partly to the development of drug resistant strains of coccidia. Furthermore, several chemotherapeutic agents have been found to leave residues in the meat, making it unsuitable for consumption.
U.S. Pat. Nos. 4,438,097; 4,639,372; 4,808,404; 5,055,292; 5,068,104; 5,387,414; 5,602,033; 5,614,195; 5,635,181; 5,637,487; 5,674,484; 5,677,438; 5,709,862; 5,780,289; 5,795,741; 5,814,320; 5,843,722; 5,846,527; 5,885,568; 5,932,225; 6,001,363 and 6,100,241 relate to coccidiosis vaccines, including live and recombinant vaccines. However, there are problems with existing coccidiosis vaccines, such as reduced efficacy, cross-infection with other parasites (e.g., Clostridium spp.) and poor bird performance. Thus, there exists a need for efficacious coccidiosis vaccines with reduced or non-existent cross-infection that do not adversely affect bird performance.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.